---
Looks like a glaring mistake in designing their trading system. You do not submit another order intended to replace an old order until that old order is confirmed cancelled by the exchange.

The mistake likely multiplied their losses by more than 10x.

BTW interesting tidbit:
"UBS handles most of the order flow from Charles Schwab Corp, one of the biggest U.S. brokerages, with about $1.8 trillion in client assets. It also takes orders from other retail brokerages, including TD Ameritrade and Fidelity."